I travel quite a bit so I try to pick up a beach themed souvenir ornament from each city I visit. This way not only am I bringing back a little something to remember my trip by, I can decorate my Christmas tree in a beach theme as well.
If you don't travel much to collect ornaments from different cities you can make them for really cheap. I have bought clear balls, poured beach sand and small shells in them and hung them on my tree. You can purchase starfish and sand dollars at your local craft store or online, drill a hole through them and stick raffia through the hole and hang them on your tree.
I recently purchased starfish garland from a local beach shop. I could have made it but it was so cheap it would have cost the same to make it. It is about a dozen smaller (approx 2") starfish that are on a string of raffia. I will hang this around my tree as garland. You could also make the same garland using seashells or sand dollars. A small drill and some raffia and you are set.
For lighting my tree, I wrap flip flop lights around the palms. Other choices that you could use are surfboard lights, blue lights, tropical shirt lights or palm tree shaped lights.
I will go to the fabric store and purchase some kind of tropical fabric to wrap around the bottom for a tree skirt. Sometimes finding tropical Christmas fabric is a little hard to find in a local craft store so I would recommend using Ebay. They always have many choices and it is inexpensive as well.